Field of Invention
The present invention relates to a flyback power converter and a control circuit thereof; particularly, the present invention relates to such a flyback power converter and a control circuit thereof, which can perform over-temperature protection (OTP) or detect whether the flyback power converter operates in a continuous conduction mode (CCM) or a discontinuous conduction mode (DCM) without requiring an additional pin.
Description of Related Art
FIG. 1 shows a conventional flyback power converter 100. An alternating current (AC) voltage Vac is rectified by a rectifier circuit 10 to generate an input voltage Vin. A primary winding W1 of a transformer 103 receives the input voltage Vin. A power switch PS controls the conduction time of the primary winding W1 to generate an output voltage Vout at the secondary winding W2. The power switch PS is controlled by a control circuit 105; the control circuit 105 is an integrated circuit which has a predetermined limited number of pins. To perform feedback control, the control circuit 105 needs to obtain current and voltage information. The current information is obtained through a current sense pin CS from a current sense circuit 107. The voltage information is obtained through a voltage feedback pin FB, from a voltage divider circuit (not shown) connected to an auxiliary winding W3 or from an opto-coupler (not shown) connected to the secondary side of the transformer 103. The control circuit 105 generates a control signal which is transmitted through a switch control pin GATE to control the operation of the power switch PS. In addition to the above pins, the control circuit 105 needs to be connected to a power source and ground, so it must have a power pin VDD and a ground pin GND (the power for example may come from the auxiliary winding W3). As such, there are five pins whose functions have already been predetermined.
In order to protect the flyback power converter 100 so that it will not be damaged by high temperature, the control circuit 105 needs to receive a signal which is temperature-related, so that the control circuit 105 can respond to temperature and shut down the power switch PS at a proper timing. In addition, in certain applications, it is required to detect whether the flyback power converter 100 operates in the CCM or the DCM. For such over-temperature protection (OTP) function and mode detection function, the control circuit 105 requires external information, that is, the control circuit 105 needs to communicate with an external circuit through its pin. Typically, the control circuit 105 is an integrated circuit having six pins. Hence, with five pins already used, there is one pin DMAG left. However, this pin DMAG is usually reserved for a function other than OTP or mode detection. Therefore, in prior art, either OTP and mode detection functions can not be achieved, or the control circuit 105 has to use a package with a higher number of pins, which is more costly.
In view of the above, the present invention proposes a flyback power converter and a control circuit thereof, which can perform OTP or mode detection without requiring an additional pin.